Linearized trans-impedance amplifier

ABSTRACT

The disclosed systems and methods utilize an advanced linearized trans-impedance amplifier (ATIA) that allows for the recovery and amplification of low amplitude analog and digital signals. This disclosure further describes unique approaches of addressing issues inherent in the transmission and reception of small amplitude multi-carrier signals used for distribution of voice, video, and data communications over both fiber optic cables and free space transmitters.

PRIORITY INFORMATION

The present application is a continuation of U.S. application Ser. No.10/993,969 filed 19 Nov. 2004, which claims priority under 35 U.S.C.§119 (e) of U.S. Provisional Patent Application No. 60/527,587 filed 5Dec. 2003; the entire contents of both of which applications areincorporated herein by reference.

STATEMENT PURSUANT TO 37 CFR §1.71(E)

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

FIELD OF THE DISCLOSURE

The present disclosure relates to circuits for the recovery andamplification of low amplitude analog and digital signals. Thedisclosure further relates to circuits used for the recovery andamplification of analog and digital signals modulated onto carriers andtransmitted via optical fiber or free-space transceivers, and to anadvanced linearized trans-impedance amplifier (ATIA) that allows for therecovery and amplification of low amplitude analog and digital signals.

BACKGROUND OF THE DISCLOSURE

Pronounced as “coming soon” for over a decade, the ability to run “lastmile” fiber optic cables for communications and data transfer has neveractually materialized except in limited field tests. The slow deploymentowes to the high component and system costs due to both expensivemanufacturing/design techniques and inadequate component performance.Thus, a technically feasible albeit brute force solution has failed tobe implemented in the face of economic realities.

Past and present attempts to implement a complete fiber optic networkare best summarized as follows:

-   -   Initial optical fiber deployments were initially limited to the        major “trunk lines” connecting large populations and data        sources due to costs for buying, laying, and connecting the        fiber to the existing communications infrastructure.    -   Follow-on deployments saw fiber optic cables extended from these        major access points outward to local distribution points, but        still not to each individual household.    -   At present, the industry has used a Passive Optical Network        (PON) design which has enabled the amortization of the cost of        the expensive optics at the Optical Line Termination (OLT) over        several homes, but the number of optical signal splits are        limited by the need to deliver Analog Video services over the        fiber. The ability to deliver analog video services is required        if the optical fiber systems is to compete with the existing        cable TV infrastructure.

Pundits and futurists have cited several uses for the impressive datatransmission capacity inherent in fiber optic based systems if such asystem were broadly deployed all the way to the end user. However, nonehas proven to be a compelling business market due to present dayeconomics of the required infrastructure. Such things as real timestreaming digital video delivered on demand could have a pronouncedability to change or open new markets if only a technical solution couldbe cost effectively delivered.

Major issues confronting the delivery of these services of a fiberdelivery based system include:

-   -   Cost of deployment vs return on investment (ROI),    -   Bandwidth limitations due to passive loss and dispersion in the        optical path, and    -   Number of customers served on a PON due to Analog receiver        sensitivity vs carrier to noise ratio (CNR).

Thus, there is a need in the art for systems and methods through whichservice providers may deploy low cost fiber systems for the massdelivery of the broadband services that the end users desire.

One stumbling block to the deployment of fiber to the home has beenlowering the cost of an optical network termination (ONT) in acustomer's house. FIG. 3 shows an example of an optical network topologyusing both powered and passive optical components. One ONT is requiredat each termination, as shown by the houses in the drawing. FIG. 4 showsa more detailed view of an example termination at a house, with the ONTclearly shown. The ONT in this example includes several parts: anoptical receiver, a wavelength-division-multi/demultiplexer (WDM)transceiver module to split a multiplexed signal into discrete channels,interface circuits to the customer terminals, and optionally, a powersupply and battery. Typically, the optical receiver is integrated withthe WDM transceiver as a single field replaceable optical WDMtransceiver module. The module provides a complete interface between theoptical transmission world and the electrical transmission world in asingle package.

Optical receivers for light guide systems of the type employed in cablesystems generally use a high frequency photodiode to convert the lightsignal to a photocurrent. The photocurrent is proportional to thereceived intensity of the light signals, and then applies the resultingcurrent to an input circuit.

Since the current obtained from the sensor circuit is often too small tobe usefully applied to data recovery circuits, it is desirable toamplify the sensor's photocurrent signal in order to make it relativelyinsensitive to the introduction of ambient noise during signalprocessing stages. To this end, optical receivers include a so-called“front end” trans-impedance amplifier (TIA) which raises the level ofthe signal several orders of magnitude. The output of the front end isthen further amplified and shaped in a later section of the signalprocessing system.

It is desirable for an optical receiver to have a wide dynamic range,both in input intensity and for input frequency. The reasons a widedynamic range of input intensities is desirable include (1) variationsin the assorted cable lengths and multiple types of light sources withwhich the optical receiver may be used; and (2) variations in lightattenuation that can occur with variations in cable lengths, all ofwhich has an effect on light intensity output. However, since the lightsignals may have significant variations in intensity, resulting in awide range of input currents, the amplification circuits need to becapable of handling a wide dynamic range of input currents depending onthe strength of the received light signal. The received signal strengthwill vary, for example, as a function of distance from the transmitter,quality of circuit components, number of passive splits, etc. In mostcases, the receiving system has no prior knowledge of its distance fromthe transmitter and topology and it is therefore important that anydesigns have the flexibility to accommodate the full range of inputsignal strengths.

An optical receiver with a wide dynamic range of input frequencies mayhandle additional channels within a multiplexed signal, or may supporthigher data rates. A limiting factor in the optical receiver's dynamicrange is the dynamic range of the trans-impedance amplifier, which is inturn limited by factors such as components selected, the circuit design,and the ambient noise introduced by various circuit components. One suchlimiting factor is the use of feedback designs, which limit the overalldynamic range of the circuit by introducing stabilization timingconstraints within the feedback loop.

In general, analog video delivery over fiber is widely used in the CATVindustry to distribute video service between Head-ends, Hubs and Nodes.As such, it would seem to be a normal evolution of system design toextend the distribution of this type of video to the home over a PONsystem using part of the 1550 nm band.

Today, state of the art video receiver designs allow the ONT/HomeGateway to receive a video signal as low as −6 dBm while stillmaintaining acceptable CNR and distortion performance over the specifiedrange of received input power. Although this gives performance levelsthat approach today's cable equivalent, it does come with a price.

Currently, optical video receivers do not use the same method ofcoupling or signal recovery for analog video transmissions as those usedfor digital signal transmissions. Digital systems use a high gainTrans-Impedance Amplifier which provides good noise performance but poorlinearity for the number of signal channels needed (79 NTSC channels) tosatisfy minimum expectations. To effectively compete with the CATVmarket, an additional 30 or more digital channels will be required to betransmitted above 550 MHz, the current limit.

One of the most efficient designs currently available uses a transformercoupling that matches the amplifier input impedance to the couplingresistor in series with the Photo-Diode as shown in FIG. 2. This type ofreceiver provides good performance for both CNR (about 48 dB) andIntermod-Distortion (approximately 0 to −5 dBm); and there is aninherent loss due to a maximum power transfer of approximately 3 dB.This loss could be decreased by coupling the current directly in to theamplifier, but linearity then becomes a greater concern at the higherlevels in received optical power.

It is additionally desirable to limit the amount of noise introducedinto the sensor circuit by the “front-end”. Limiting the amount ofintroduced noise permits the circuit to operate at lower power and withhigher responsiveness, providing advantages such as more efficientsignal recovery, reduced operating costs, less heat dissipation, andimproved dynamic range.

It will be appreciated that improvements in the art described in thepresent disclosure that satisfy the above requirements will find use inother fields in which it is necessary to recover a low power signal froma carrier signal, or to amplify a low power signal for furtherprocessing.

SUMMARY OF THE DISCLOSURE

The following discussion of advantages is not intended to limit thescope of the invention, nor to suggest that every form of the inventionwill have all of the following advantages. As will be seen from theremainder of this disclosure, the present invention provides a varietyof features. These can be used in different combinations. The differentcombinations are referred to as embodiments. Most embodiments will notinclude all of the disclosed features. Some simple embodiments caninclude a very limited selection of these features. Those embodimentsmay have only one or a few of the advantages described below. Otherpreferred embodiments will combine more of these features, and willreflect more of the following advantages. Particularly preferredembodiments, that incorporate many of these features, will have most ifnot all of these advantages. Moreover, additional advantages, notdisclosed herein, that are inherent in certain embodiments of theinvention, will become apparent to those who practice or carefullyconsider the invention.

One advantage of the presently disclosed apparatus and method overexisting apparatus and methods is the reduction in paid-indollars/milli-watt of optical power generated at the Head-end or centraloffice (CO) due to the use of high power Erbium doped fiber amplifiers(EDFAs) at the Hub or Head-end for distribution of the 1550 nm videosignal. By increasing the sensitivity of the video receiver withoutsevere degradation of the CNR and distortion performance, the presentapparatus and method lower the cost of the video distribution by eitherallowing a decrease in EDFA output power or an increase in homes passedper high power EDFA.

In a second advantage, the disclosed apparatus and method will allow theintegration of the analog receiver function into a package with a photodiode for better control of the RF matching and coupling. This willincrease the performance of the analog optical receivers, allowinglonger reaches, lower EDFA output power, and the distribution of thesignal to more end users. In general, this has the effect of reducingthe amount of optical power needed for signal distribution in networksor allowing for an increase in the potential revenue for dollars spenton the generation of the optical signal in existing infrastructures.

Eventual integration of this technology into silicon optical bench orother semiconductor processes that can also be used to fabricate lasersand photodiodes will allow smaller, lower power components enablingfurther applications which could include delivery of voice, data videoservices to a PC, home Entertainment Centers, Flat panel TV's, Highspeed internet Gaming devices, etc.

Further advantages of several other embodiments of the disclosedapparatus and method include:

-   -   Greater linearity, resulting in lower harmonic noise introduced        into the circuit,    -   Greater amplification of low level signals while maintaining        superior CNR and Distortion performance,    -   Increased dynamic range of Analog Fiber Optic Receivers by at        least 3 dB-6 dB,    -   Reduced component count,    -   Decreased size of the Analog Receivers,    -   Eliminates the use of passive components such RF transformers        for coupling,    -   Reduced energy dissipation, permitting more efficient packaging        and reduced manufacturing costs,    -   Allows for the placement of the amplifying device in the same        package as the Photodiode and optics used to couple the light to        the photodiode, and    -   Provides the technology for Analog Video that would allow the        ATIA receiver and Photodiode to be fabricated as a single        device.

A primary advantage of the disclosed ATIA is that it provides a simplecircuit implementation that achieves the desired goals of: a) operatingwith a low input bias current; b) wideband normalization of the inputsignal; c) fast overload recovery; d) good accuracy for DC and widebandsignals; e) simple implementation of automatic gain control; and f)stability for a wide range of input conditions.

The disclosed ATIA is a circuit that has both high current gain andexcellent linear properties to enable the recovery of extremelylow-level analog signals modulated upon a carrier signal beyond thecapabilities of other circuits here-to-for discussed. Along with highgain and low noise characteristics, feed forward gain control techniquesprovide for use of the circuit as a Trans-Impedance Amplifier (TIA) withimproved response and dynamic range. This circuit can be used within theAudio (10 Hz) to Microwave (10 GHz) frequency range.

In an example of one preferred embodiment an ATIA with automaticfeed-forward gain control (AFFGC) includes an optical (photo-diode)sensor. An alternative preferred embodiment provides further reducednoise and increased linearity by stabilizing the sensor photo-diodecoupling stage bias current. Stabilizing the bias current through thetransistors in the first coupling stage improves upon bothinput-referred noise or equivalent input noise (EIN) and linearity ofthe transistors in this stage when operating at the higher input opticalpowers. This improvement can be quantified as an increase in the dynamicrange of the ATIA by 2 dB (from −1 dBm to +1 dBm of optical inputpower). This is achieved because the bias control keeps the transistorsin their most linear region throughout the input range of the ATIA. Theimprovement in noise is achieved by limiting the bias current throughthe transistors when the optical input power reaches −3 dBm to +1 dBm.

In an alternative preferred embodiment, the basic ATIA with AFFGCcircuit disclosed herein is changed only in the fact that there is DCfeedback to the emitter of the first transistor of the sensor couplingblock. Additional (optional) circuit blocks named “VCCS Control” and“VCCS” are added to provide this feedback. This circuit is shown in FIG.12 and is described hereinafter. It will be appreciated that biascurrent stabilization as described herein allows this type of couplingto be used with Field Effect Transistors (FETS).

An additional benefit of the alternative preferred embodiment shown inthe circuit diagram in FIG. 12 is the same for the different processtechnologies as it is for a bipolar junction transistor process. Itenables the devices to reflect a lower Equivalent Input Noise (EIN) overother approaches by eliminating the extra noise reflected to thebase/gate of the first transistor of the coupling stage and creates amuch lower apparent input resistance for the additional sensor currentcaused by the DC component of the optical signal applied to the sensor.An added benefit is the removal a linear component (resistor) togenerate the AC voltage needed to couple in to a base or gate of thefirst stage transistor of the TIA or amplifier. This additionalcomponent has both the AC and DC currents from the sensor passingthrough it creating more input noise which cannot be eliminated usingthe accepted coupling technique, its elimination reduces the input noiseassociated with this component.

The foregoing and other benefits are achieved by the apparatus andmethods described herein which overcome problems inherent in systemsemploying optical transmission techniques which include, but are notlimited to, optical networks using distribution methods with largeamounts of passive coupling loss or through air optical transmission.Other objects, advantages and novel features, and further scope ofapplicability of the present invention will be set forth in part in thedetailed description to follow, taken in conjunction with theaccompanying drawings, and in part will become apparent to those skilledin the art upon examination of the following, or may be learned bypractice of the invention. The objects and advantages of the inventionmay be realized and attained by means of the instrumentalities andcombinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed apparatus and method are described in detail withreference to the accompanying drawings. Each of the figures is aschematic diagram more fully described below.

FIG. 1 shows a video signal transmissions in a typical passive opticalnetwork (PON) which can be used to deliver voice, data and video to anend user.

FIG. 2 shows the typical circuit topology and design currently used inanalog video recovery in cable TV (CATV) and Fiber-to-the-Home (FTTH)optical networking components.

FIG. 3 represents an example of a network architecture for connectinghomes (end-users) to the fiber optic network using the teachings of thepresent disclosure.

FIG. 4 is a further schematic of the fiber optic link into the homeusing the teachings of the present disclosure.

FIG. 5 shows the preferred coupling technique in accordance with theteachings of the present disclosure, the technique providing excellentnoise performance without the coupling losses, but is inherently nonlinear with regard to traditional trans-impedance amplifier (TIA) andcoupling approaches.

FIG. 6 is a block diagram of an Optical Triceiver that uses thelinearized TIA technology described in the present disclosure to recoverBroadcast Video signals transmitted in FTTx systems.

FIG. 7 is a block diagram of an analog TIA using a specific linearizeddesign approach which reduces the inherent non-linearity's of the TIAenabling the use of the preferred coupling technique of FIG. 3.

FIG. 8 shows a block diagram of an ATIA circuit designed in accordancewith the present disclosure.

FIG. 9 details the reduction in size and component count within a modulewhich will be afforded through the incorporation of disclosed apparatusas compared to the current state of the art.

FIGS. 10 a and 10 b show block diagrams of common receiver modules usinga preferred embodiment of the disclosed apparatus.

FIG. 11-11F is a schematic of a preferred embodiment of a basic ATIAcircuit, including a photo-diode sensor.

FIG. 12-12H is a schematic of an alternative preferred embodiment of anATIA circuit with optional bias control, including a photo-diode sensor.

FIG. 13 is a schematic of an alternative embodiment of a sensor blockfor voltage producing sensors.

FIG. 14 is a schematic of an alternative embodiment of a FET-basedimplementation of a system comprising a sensor block, sensorcoupler/voltage conversion block, VCCS control block and VCCS block.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In regards to communications networks and distribution schemes ingeneral, multiple disparate systems have been created in order to supplydata and other communication services to end-users. As such, a typicalhome will have to incorporate multiple receivers in order to accommodatethe receipt and/or transmission of data for each of these differentsystems. These systems can include phone and fax services, analog anddigital, one-way and two-way video services, IP-based data services,etc.

With the use of an advanced ATIA, multiple types of services can beoffered at a cost competitive rate with other delivery mechanisms usinga single unified delivery system. This unified delivery system will mostlikely support a mixture of:

-   -   45-862 MHz one-way cable television.    -   5-65/85-862 MHz full duplex cable television, permitting the use        of DOCSIS compliant cable modems. DOCSIS is the ITU endorsed        “data over cable systems interface standard” as defined by US        based Cable labs. The service also supports the DVB based        Euromodems, legacy IEEE 802.7 based equipment for sub-split        networks, and RF modems for video on demand (VOD) applications        and the likes.    -   10/100/1000 Mbps full duplex Ethernet (10/100/100BaseT).    -   Plain old telephone services (POTS).    -   Integrated services digital network, basic rate interface        (ISDN-BRI).    -   Integrated services digital network, primary rate interface        (ISDN-PRI).    -   Data interface for residential alarm panel or telemetry.    -   Digital and/or analog audio data.

To overcome the problems which have up until now prevented the adoptionof fiber to the end user, the disclosed apparatus and method:

-   -   Offer a more sensitive Analog Video receiver technology to        maintain good CNR.    -   Provide optical modulation techniques for data transmission        similar to DSL to enable greater distances from the OLT and        increased Tx to Rx optical split ratio.    -   Affords Digital Video Transceivers for both OLT and ONT        applications.

In a preferred embodiment, shown generally in FIG. 5, and in more detailin FIGS. 11 and 12, the disclosed coupling of a photo-diode sensor tothe amplification stage provides significant benefits. By using a lownoise process such as SiGe and an ATIA, the noise contribution of theamplification circuits can be kept low. This allows the recovery of thelower level signal while maintaining a respectable CNR, and eliminatingthe coupling losses due to the impedance matching circuit.

To help improve linearity and save in device power consumption,pre-distortion techniques can be used by designing a pre-amplifier andpost amplifier stage with equal and opposite, or complementarydistortion characteristics. As seem from the functional diagram of theAnalog TIA (ATIA) shown in FIG. 7, this will allow higher gains to beachieved improving further in the sensitivity of the Video Receiver.

The design of a preferred embodiment provides significant cost and spacesavings, including the integration of the optical components within asingle die. This substantially reduces the cost, power consumption, andsize of the resulting module. FIG. 9 illustrates the substantial sizereduction afforded by this integration, first by reducing theATIA/coupling interface to a single die (Triplexor #1), and second byreducing the entire triplexor circuit to a single die (Triplexor #2).

This approach enables the integration of several different functionsused in Passive Optical Network data voice and video transmission. Anexample of this using a disclosed embodiment within an industry standardoptical triplexor and ONT module is shown in FIG. 6. Other examples ofindustry standard module configurations using a disclosed embodiment areshown in the block diagrams in FIGS. 10 a and 10 b. In FIG. 10 a, aunidirectional module is shown in which the module operates in areceive-only mode such as with today's CATV systems. In this example,the optical signal is received at the optical splitter, and is routed totwo different ATIA circuits based in part upon wavelength or frequencyconsiderations. In FIG. 10 b, an alternate module configuration is shownin which the ATIA is integrated within the optical transceiver and videoreceiver components and the ATIA is not provided as a separate circuitwithin the module.

The frequency range of operation of the disclosed ATIA is determined inpart by the type of sensor selected, the silicon technology employed inthe design and fabrication of this circuit and the values of componentsas described in figures. The components described herein are optimizedfor use within the preferred embodiment of an optical receiver andprovide optimum response in the frequency range of 25 Mhz to 2 GHz. Itwill be appreciated by those skilled in the art that the circuit designwill provide the best response within the 10 Hz to 10 Ghz operatingrange by optimizing the values of various resistors.

The ATIA of the present disclosure has the following advantages:

-   -   Provides highly efficient current to voltage conversion from        sensor to RF amplifiers—Achieved with fabricating IC's using low        noise high frequencies processes that enable direct conversion        of sensor output to voltage.

Provides direct control of gain by using the average optical power levelto the receiver to control gain and noise cancellation in converter.

-   -   Allows ICs and sensors to be combined in the same package so as        to reduce parasitic induced noise currents.    -   Allows IC's and sensor components to be fabricated as one piece        of silicon.

Use of noise and distortion reduction techniques results in increasedreceiver linearity.

FIG. 7 contains a functional diagram of this concept, which first willbe developed as a chipset and eventually integrated into a single chipfor integration onto silicon optical bench technologies and eventuallyfabricated with the optical components such as lasers and photodiodesused in the 600 nm-1700 nm range.

The disclosed ATIA can be produced using various manufacturing processesand materials, including the following:

-   -   Heterojunction Bipolar Transistors (HBT).    -   Field Effect Transistor (FET) technologies, including but not        limited to GaAs, CMOS, BiCMOS, InGaAsPh, and InPh.    -   SiGe.

The choice of manufacturing process is dependent on the specificationsrequired to be implemented, costs, and delivery time.

The disclosed ATIA is logically and preferably (although notnecessarily) divided into three stages with defined attributes. Thesestages include:

-   -   Coupling stage—Preferably uses dynamic impedance of the sensor        for input power detection, gain control and amplifier        stabilization—Preferably does not require RF impedance matching        which eliminates the matching power losses due to the need for        input mismatching to achieve low noise figure in the amplifier.    -   Linearization Stage—Preferably uses special biasing techniques        for the components to achieve desired function—Preferably uses        both active and passive components to get complex conjugate        functions—Preferably employs both non linear distortion and        Complex Conjugate matching to reduce both Inter-modulation        Distortion, non-linear amplifier distortion Composite Second        Order and Composite Third Order Distortion.    -   Amplifier Stage—Preferably uses special biasing techniques to        achieve the conjugate function of the linearization stage and        reduce power consumption—Preferably uses both active and passive        components to get complex conjugate functions—Preferably employs        both non linear distortion and Complex Conjugate matching to        reduce both Inter-modulation Distortion, non-linear amplifier        distortion Composite Second Order and Composite Third Order        Distortion when paired with the linearization stage.

One of the many uses of an ATIA of type described herein is forrecovering signals that require high carrier/channel content such asCATV or Satellite TV. Due to the inherent non-linearity and bandwidth ofthe amplification circuits used for current amplification of thephotodiode within the CATV industry, TIA's have never before beenconsidered for this use. Additionally, in the preferred embodiment, theATIA of the type described herein uses the dynamic impedance of thephotodiode sensor as part of the amplifier feed forward circuit, whichin turn provides the ability to exercise discrete control over severalfunctions including optical input power detection, gain control, andamplifier stabilization.

Further, the ATIA detailed within the preferred embodiment preferablyuses a three stage approach to provide linearization with regard to it'sinput/output transfer characteristics. More specifically, the ATIA usesa pre-amp stage with gain control, a pre-distortion stage, and apost-distortion stage with complex conjugate matching and gain control.This approach takes into account the noise and non-linear properties of(1) the fiber, (2) the photodiodes that are used as sensors, e.g.,Indium Gallium Arsenide/Indium Gallium Arsenide Phosphide(InGaAs/InGaAsP) photodiodes, and (3) the semiconductor process used formanufacturing the amplification stage so as to construct a linearizationcircuit that can be used for high carrier/channel content applicationsrequiring, for example, optical input powers of −9 to −12 dBm andrequiring a CNR performance of 46-50 dB and composite second order (CSO)and composite triple beat (CTB) of −56 to −60 dB.

One application of such a circuit is in the recovery of multiple RFcarrier signals transmitted over optical fiber. In a preferredembodiment, the signal is modulated using a 1260-1650 nm laser or LEDlight source. These sources can be either provide direct or externalmodulation. Each modulated signal can have 1-200 plus carrier signals,which require highly linear, high gain, low noise circuits for propersignal recovery. This type of transmission of signals is useful inseveral different end-use applications. An example of one suchapplication is where the signal is transmitted as part of afiber-to-the-premise (FTTP) system as a broadcast video overlay whichtransmits the signal using 1550-1600 nm modulation. In an alternativeapplication the signal is transmitted through a CATV system inconjunction with broadcast video signals using 1260-1600 nm modulation.In another example, the signal may be used in a satellite antenna remoteapplication of Ku, C and L band signals using 1260-1600 nm modulation.

Referring to FIG. 6, an ATIA with automatic feed forward gain control1000 (ATIA with AFFGC) comprises a plurality of sub-circuit blocks, withadditional optional sub-circuits that enable additional featuresrelating to the disclosed apparatus and method. These sub-circuits arereferred to as functional blocks.

The sensor block 1100 shown in FIG. 8 provides a sensor and circuitryfor converting sensor output into a current that varies linearly withthe sensitivity of the sensor. In a preferred embodiment for use withoptical networks, for example the PON networks described above, thesensor block 1100 comprises of a photo-diode D2, resistor R13, andinductance coil L2. The circuits for the preferred embodiments (withoutand with optional bias control) are shown in FIGS. 11 and 12. The sensorblock 1100 in the preferred embodiment produces a current that islinearly proportional to the light received at the photo-diode. In analternate embodiment, the sensor may be part of medical devices or otherproducts that have sensors to measure aspects of the environment. In asecond embodiment, the sensor is an electrochemical sensor that detectssubstances within a sample, such as blood chemical or environmentalanalysis sensor. In this embodiment, the sensor produces a currentproportional to the trace elements detected in the sample underevaluation. It will be appreciated that sensors of this type producevery low current upon detection of trace elements, and that accuracy ofanalysis of the sample under evaluation is, at least in part, a functionof the quality of the signal amplification provided by the presentimplementation.

Alternate embodiments of sensor block 1100 support sensors that providea varying voltage instead of a current, such as the circuit illustratedin FIG. 13. In this alternate embodiment, a voltage-producing sensor1302, such as piezo-electric or capacitive transducer (e.g. radioactive,vacuum, and pressure sensors), the voltage producing sensor may beconnected, in series with a resistor R1 or by capacitive coupling, tothe emitter of the sensor, with the voltage output being provided to theATIA (not shown in the Figure), and, with the use of a voltage dividerR2 and R3 provides a sensing voltage to the sensor monitor (not shown inthe Figure).

It will be appreciated that sensor block 1100 may be fabricated as partof a single die, which further reduces component count, productioncosts, unit size, and noise related to connections between components.

Again referring to FIG. 8, sensor block 1100 is operatively connected tosensor coupling and voltage conversion block 1200, is additionallyoperatively connected to the feed forward control block 1400, and isoptionally operatively connected to sensor monitor block 1500. Thesensor coupling and voltage recovery block 1200 is operatively connectedto sensor block 1100 and power gain and linearization block 1300. Sensorcoupling and voltage recovery block 1200 converts an input signal fromthe sensor block 1100 to a possibly different form of signal (e.g.current to voltage) compatible with the power gain and linearizationblock 1300.

A preferred embodiment of the sensor coupling and voltage recovery block1200, is detailed in FIGS. 11 and 12. Block 1200 preferably comprisestransistors X15 and X14, resistors R1-R4, R6, R7, R96, R65, R19, R23,capacitors C2, C3, C5, C13 and inductor L9. In the present apparatus,the input signal (from the sensor block 1100) is coupled into theemitter of transistor X15. Transistor X15 is biased in a common baseconfiguration. This combined with the emitter coupling of the sensorprovides a low impedance input with low noise and high bandwidthcapabilities.

Voltage is generated across the series parallel combination of resistorsR23, R2, R96, R4, and (Beta×R19), as shown in FIGS. 11 and 12. Thisvoltage is then transferred to the emitter of X14 to the input of thepower gain and linearization block 1300 referenced in FIG. 8.

The gain (Av1) in the sensor coupling and voltage recovery block 1200 isapproximated by:Av1=(Ipd)×(R2+R96\\R4)

-   -   Where Ipd=the AC current induced in the sensor.

In the preferred embodiment, this current is the current induced by thephoto diode 1302 (best shown in FIG. 13) in the sensor block from theoptical signal received at the photo diode.

It will be appreciated that the coupling circuit provides the ability tolower the equivalent input noise (EIN) of the TIA significantly becauseof the low load impedance for the recovered AC signal along withproviding significant gain for the first stage.

Again, referring to FIG. 8, the power gain and linearization block 1300is operatively connected to the sensor coupling and voltage recoveryblock 1200 and the feed forward control block 1400 for input, andproduces an output that is operable coupled to the impedance matchingblock 1600.

The power gain and linearization block 1300 as shown in FIG. 8, anddetailed in FIGS. 11 and 12, comprises transistors X13 and X12,resistors R49, R51, R52, R54, R56, R59, R64, capacitor C29 and inductorL10, as shown in both FIGS. 11 and 12. The feed forward control block1400 in FIG. 8 operates with the power gain and linearization block 1300to adjust its gain as a function of input sensor power to maintain thelinearity of the amplifier.

It will be appreciated that the configuration of the amplifier,specifically, by connecting the common emitter transistor X13 to acommon base transistor X12 directly from collector to emitter, providesadvantages including linear power amplification, which in turn enablesthe circuit to have adjustable gain while maintaining linearity across amuch broader operational range. It will be further appreciated thatadjusting the gain by using a derivative of the DC sensor current in theemitter of X13 further enhances the low noise characteristics of theATIA.

The gain (Av2) in power gain and linearization block 1300 isapproximated by:Av2=(R54)/(R49\\Rff)

-   -   Where Rff is the equivalent resistance of the feed forward        control block 1400.

The feed forward control block 1400, as shown in FIG. 8, provides gaincontrol to the power gain and linearization block 1300. The feed forwardcontrol block 1400 is operatively connected to the sensor block 1100 andthe sensor monitor block 1500 for input, and is operatively connected tothe power gain and linearization block 1300 to, in part, control theATIA gain provided by that block.

The feed forward control block 1400 as shown in FIG. 8, and detailed inFIGS. 11 and 12, comprises transistors X7, X8, X9, three comparators(AMP2, AMP3, AMP4), voltage reference X16 and D1, resistor R80, currentmirror C_M_1, capacitor C30, resistor ladder R81-84and gain adjustresistors R74-76. Other components include resistors R77-R79, andR85-R91, and capacitors C31-C33. It will be appreciated that transistorsX7-X9 are used in place of RF switches for gain adjustment to improvelinearity and bandwidth performance to levels that could not be achievedusing traditional RF switches or other current controlled resistivedevices such as variable resistance RF diodes.

The comparators control the switching points of each transistor,switching the transistors between their on and off states. Alltransistors (X7-X9) that are in the on state provide maximum gain, andconversely all transistors in the off state provide minimum gain. Theresistor ladder R81-84 of FIGS. 11 and 12 provides the switch pointvoltage for each comparator. Each switch point voltage represents asensor input power value that the gain switch should occur. The valuesof resistors R81-R84 may be altered to further tune the presentapparatus for specific applications, although it will be appreciatedthat no alteration or additional tuning is required for the presentinvention to perform within the broad frequency range of 10 Hz to 6 GHz.

In the disclosed apparatus, the current mirror C_M_1 in feed forwardcontrol block 1400, detects the current through sensor 1100. Thiscurrent is directly proportional to the input power. In a preferredembodiment where the sensor is a photo diode, the current isproportional to the input optical power and is determined by the quantumefficiency of the photo diode.

So:Icm=(Quantum Efficiency A/W)(Input Optical Power W)

-   -   Where Icm is the detected current mirror current;    -   A/W is the Quantum Efficiency;    -   A is ______; and    -   W is the Input Optical Power

In the preferred embodiments, a current mirror circuit C_M_1, asdetailed in the feed forward control block 1400 of FIGS. 11 and 12 isused to provide gain to the ATIA circuit. It will be understood by thoseskilled in the art that a current mirror circuit such as the one shownin the figures provides advantages over traditional methods such as aresistor of providing current gain, such as ease of manufacture in avariety of silicon technologies (e.g. FET, MOS). It will also beappreciated that a current mirror design such as the one describedherein provides superior noise reduction using fewer components, furtherproviding the advantages of lower cost to manufacturer, reduced powerconsumption, and a smaller silicon die footprint.

The sensor current mirror employed in the present apparatus is amodification of a Wilson Current Source/mirror. A resistor in the baseof the transistor generating the reference current for the currentmirror is used to generate a voltage that is directly proportional tothe DC value of the sensor current produced by the sensor block 1100.This makes the current relationship slightly more linear than othertraditional methods of attaining current gain.

In traditional prior art current mirror designs, the two transistors ofthe current mirror must be matched on a single die. In the preferredembodiment, the current mirror is used to provide current gain, somatching of the current mirror transistors on a single die is notrequired, but the same transistor type and model should be used for thecurrent mirror. This reduces manufacturing complexity and part cost. Itwill be further appreciated that the current mirror configuration asused in the present apparatus is constructed using mismatched resistorswithin the current mirror, with the amount of current gain produced bythe current mirror being adjustable by adjusting the relative values ofthese resistor.

This current mirror current is then converted to a voltage by pullingthe current through resistor R80 shown within feed forward control block1400 of FIGS. 11 and 12 and an internal small resistance. The voltagegenerated by R80 is used by the gain comparators and the voltagegenerated using the internal resistance is used by the sensor monitorblock.

The following details the formula for the current gain Ai:Ai=Is2[e^VBEX1(39.1)(1+VCBX2/VAF)]/ICM

Where;VEBX1=0.025581n((ICM/Is1)+1)+ICM(RE1)−IC2(RE2)IC2=Is2[e^VBEX1(39.1)(1+VCBX2/VAF)]

-   -   Is=PN Saturation Current    -   VAF=Forward Bias Early Voltage

The sensor monitor block 1500 provides a voltage output proportional tothe average current level, which is operatively linked to the currentmonitor to, in part, adjust the input to the gain control circuitry. Thesensor monitor block 1500 also provides a test point for externalmeasure of power provided at the sensor. In a preferred embodiment, thistest point provides an external measurement point of the optical powerreceived at the photo-diode sensor.

The sensor monitor block 1500, as shown in FIG. 8 and detailed in FIGS.11 and 12, comprises a non-inverting amplifier with a gain that isdetermined by resistance values R9 and RIO. This gain is equal to:Apdm=(1+R9/R10)

-   -   Where Apdm is gain of sensor monitor block 1500.

The impedance matching block 1600, shown generally in FIG. 8, and inmore detail in FIGS. 11 and 12, matches the impedance of the externalcable or trace to minimize reflections and other circuit-induced noise.The preferred impedance matching block 1600 comprises transistor X11,resistors R15, R17, R28, R29, R58, R66, capacitors C8, C37 and inductorL8. Transistor X11 provides a buffer from a high impedance collector toa low impedance high capacitance load such as 50 Ohm or 75 Ohmtransmission line with the proper termination resistance. The passivecomponents connected to the emitter of X11, along with the biasing fortransistor X11, form a termination network for both forward path andtransmit/return loss.

The optional bias control stabilization circuit is generally shown inFIG. 8 as an optional block which may be, but does not necessarily haveto be used. The optional circuit comprises two additional functionalblocks; a Voltage Controlled Current Source (VCCS) Control block 1700and the VCCS block 1800.

The VCCS control block 1700 is an optional block that controls the VCCScircuit using the output of the sensor monitor block 1500 as its input.It consists of an amplifier AMP6, resistors R98-100, R16, and R14, andcapacitors C9 and C47.

The VCCS block 1800 is a well known, widely used circuit blocktherefore; it will not be explained in detail here. The VCCS block 1800is operatively coupled to the VCCS control and the sensor block 1100. Insome physical implementations, the VCCS block may be combined with itscontrol block and the sensor block. The basic function of the VCCS block1800 is to vary the sourced current with a particular voltage to currentratio when a control voltage is applied. In one embodiment this block isdesigned to have a voltage to current gain ratio of about 0.001. Detailsof the preferred embodiment of the block 1700 and 1800 are shown in FIG.12.

An alternate embodiment includes constructing the disclosed system usingFET technology, the latter including but is not limited to: GaAs, CMOS,InGaAsPh, and InPh technologies. Implementation of the describedtechnology requires extra care to keep the bias current stable or thetransconductance (gm) because changes in the bias current dramaticallycauses changes in the gate-to-source voltage (Vgs) with drain-to-sourcecurrent (Ids). The bias point also effects the values of gain-to-sourceresistance (Rgs) and drain-to-source resistance (Rds), which change boththe gain and the frequency response of the circuit.

The circuit in FIG. 14 shows one preferred embodiment of the couplingstage using FET technology. In the FET version of the described ATIA,consisting of Resistors R1, R2, R4, R5, R7, R8, R10, R11, R12, R14, R16,R17, R18, R19, R20, R21, R22, OpAmps AMP2 and AMP3, photo-diode X1,capacitors C3, C4, C7, C8, C9, C10, C11, inductor L1, and transistors Q6and Q7, the first transistor (coupling transistor) is biased at aspecific Vgs which is dependent on the transistor type (enhancement ordepletion mode). This produces the specific Rgs needed to limit both theEIN and maximize the frequency response of the ATIA. The operation ofthe bias control circuit is the same as what is used for the bipolarjunction version of the described ATIA.

The VCCS Control block 1700 preferably contains a voltage reference anda difference amplifier as shown in FIG. 12. The difference amplifier isconfigured to have a gain of one and a frequency bandwidth of 16 KHz.The bandwidth is limited to avoid any gain oscillation in the operatingfrequency band. The reference voltage is set to the optimum value forthe collector current of the VCCS which provides the best EIN andlinearity for the sensor coupling and voltage recovery block 1200'sdynamic range. In the alternative preferred embodiment shown in FIG. 12,this reference voltage is determined by a resistor divider, but thereference voltage may be supplied by any number of different methods ofproviding a voltage reference. A reference voltage, for example, can beprovided to input one of the difference amplifier, while the output ofthe sensor monitor block 1500 can be connected to input two of thedifference amplifier (AMP6). With this arrangement the output voltage ofthe VCCS control block 1700 decreases as the sensor monitor 1500 outputvoltage increases. This then generates a voltage that makes the VCCScurrent inversely proportional to the sensor current which enablesstabilization of the bias current in the coupling stage. It will beappreciated that this substantially improves the performance ofphoto-diode sensors as shown in the preferred embodiments of the presentdisclosure.

These advancements to the state of the art are a non-trivial exercise,and require exemplary knowledge of: a) optical transmission componentdesign and properties, b) high level RF design techniques, and c) howboth sensor and amplifier non-linearity's effects signals using bothvestigial sideband and QAM/QPSK modulation are necessary to create thelinearization circuits necessary for this invention.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

Definition List Term Definition ATIA Analog Trans-ImpedanceAmplifier(TIA), Linearized TIA CATV Cable Television CNR Carrier toNoise Ratio CO Central Office(Building used to House Switching andTransmission equipment for a Telephone Company) EDFA Erbium Doped FiberAmplifier EIN Input-referred noise or EIN (Equivalent Input Noise) isthe noise voltage or current that, when applied to the input of thenoiseless circuit, generates the same output noise as the actual circuitdoes: This value is very important parameter when determining the signalto noise ratio of detected low level analog or digital signals. FTTHFiber To The Home Forward Bias Early The Early voltage of the forwardbias current Voltage Head-end Building used in CATV distribution thathouses the Satellite Receivers and Optical transmitters for Transmissionof the TV Video Signals. It also has the Voice and Data switching EQ.Hub Building used in CATV distribution that houses the Opticaltransmitters for Transmission of the TV Video Signals. It also has theVoice and Data switching EQ. NTSC National Television StandardsCommittee ONT Optical Network Termination, Box used in the FTTH systemthat attaches to Office building or home. This Box converts the Opticalsignals to the Voice, Data and Video signal used in the home or office.PON Passive Optical Network SiGe Silicon Germanium- High frequency lownoise semiconductor process used heavily in Wireless Systems. TriceiverOptical device which includes three functions and transmits and receivesusing two or three wavelengths. It is used for Bidirectional datatransmission with single or dual wavelengths and reception of a thirdwavelength for broadcast of wideband analog or digital information.

1. A circuit for use in an optical network for transmitting in a rangebetween and including 10 Hz to 6 GHz, the circuit comprising: aphotodiode for sensing an input optical signal, and providing a outputcurrent signal as a function of the input signal, and a trans-impedanceamplifier connected to the photodiode for amplifying the output of thephotodiode, wherein the trans-impedance amplifier comprises a currentmirror, wherein the trans-impedance amplifier has a gain as a functionof resistor values used in the current mirror.
 2. A circuit according toclaim 1, wherein the circuit is incorporated into a single die.
 3. Acircuit according to claim 1, wherein the circuit is configured andarranged as an upstream component.
 4. A circuit according to claim 1,wherein the circuit is configured and arranged as a head end.
 5. Acircuit according to claim 1, where the circuit is configured andarranged as a module in a passive optical network.
 6. A circuitaccording to claim 1, wherein the trans-impedance amplifier furthercomprises a feed forward gain control for providing relatively broadbandwidth.